Lipids refer to substances that have a long-chain fatty acid or similar hydrocarbon chain in a molecule, and that are present in an organ or derived from a zoic organ. Lipids can be classified into simple lipids and complex lipids. Simple lipids are composed of C, H and O, and are generally soluble in acetone; the simple lipid triacylglycerol is present as an energy reservoir in a fat tissue of an animal body. On the other hand, complex lipids are a group of lipids containing P of phosphoric acid, N of a base etc. Therefore, complex lipids are composed of a hydrophobic part (fatty acid part) and a hydrophilic part (phosphoric acid and base parts), and exhibit an amphophilic nature. Generally, the above simple lipids are soluble in acetone, while the complex lipids are insoluble in acetone. Such complex lipids are a constituent of biomembranes.
The above complex lipids can be classified into (1) glycerophospholipids (such as phosphatidylcholine (also known as lecithin), phosphatidylethanolamine, etc.), (2) sphingophospholipids (such as sphingomyelin, ceramide ciliatine, etc.), (3) sphingoglycolipids (such as cerebroside, sulfatide, ganglioside, etc.) and (4) glyceroglycolipids (such as lipids in which various saccharides bond to diacyl glycerol existing in microorganisms or higher plants). The above (2) sphingophospholipids and (3) sphingoglycolipids are generically referred to as “sphingolipids”.
The above glycerophospholipid is a generic term for lipids having glycerophosphoric acid backbone in their structure, and includes phosphatidylcholine (lecithin), phosphatidylethanolamine, diphosphatidylglycerol, etc. Many lipids belonging to this glycerophospholipid are those in which the non-polar portion is a fatty acid ester, while some are of a plasmalogen form having a vinyl ether bond.
The above glycerophospholid is important as a constituent of biomembranes; above all, the plasmalogen-form glycerophospholipid has high radical sensitivity due to its vinyl-ether bond, and, hence, in recent years has been highlighted as a phospholipid having anti-oxidation nature. It has recently been reported that the plasmalogen-form glycerophospholipid contributes to oxidation-stability of phospholipid membrane containing cholesterol through a mechanism different from α-tocopherol (vitamin E) that is an anti-oxidation constituent of cell membrane (for example, see Non-patent Literature 1). Further, it is also highlighted that the plasmalogen-form glycerophospholipid not only takes part in the oxidation resistance of cell membrane and lipoprotein, but also has an important role in the information communication system of cells (for example, see Non-patent Literature 2).
The above plasmalogen-form glycerophospholipid is expected to have the function of preventing the death of brain nerve cells in dementia. Under the circumstances, however, no supply source that is safe and makes a large amount available has been found.
On the other hand, sphingolipid is a generic term for lipids having a long-chain base such as sphingosine, and it is composed mainly of sphingoglycolipid and sphlngophospholipid as described above. The sphingoglycolipid contains a long-chain base such as sphingosine, phytosphingosine etc. in addition to saccharide and long-chain fatty acid. The simplest sphingoglycolipid is cerebroside; apart from cerebroside, sphingoglycolipid includes sulfatide in which a sulfuric acid group is bonded thereto, ceramide oligohexoside in which several molecules of neutral saccharide are bonded thereto, ganglioside in which sialic acid is bonded thereto, etc. These lipids are present in the cell cortex, and are thought to take part in a recognitive mechanism.
Sphingophospholipids are classified into derivatives of ceramide 1-phosphoric acid and derivatives of ceramide 1-phosphonic acid. Sphingomyelin is a well-known example of the former, while ceramide ciliatine (ceramide aminoethylphosphonic acid) is a well-known example of the latter.
These sphingolipids are highlighted since it has been shown in recent years that ceramide, sphingosine, sphingosine-1-phosphoric acid, etc., which are decomposition metabolites thereof, take part in the information communication in cells. Further, sphingolipids take part in the formation of a membrane microdomain called a “raft” together with cholesterol, etc., and it has been shown that this microdomain plays an important role as a site of information communication; accordingly, further increased attention has been paid to sphingolipids.
These sphingolipids have hitherto been extracted from cow brains and utilized; however, those that are derived from cereals or fungi are now used from a safety standpoint. Since, however, sphingoid bases constituting sphingolipids derived from cereals or fungi differ from those of mammals, there is a problem that their utility in organisms is low compared with human-form sphingolipids.
Meanwhile, when a relatively large amount of sphingomyelin is produced from total lipids of foods or animal tissues, it is produced by eluting it stepwise by means of column chromatography using silicic acid, etc., or by fractionating it stepwise according to a solvent fractionation method. Both of these require complicated procedures. In the solvent fractionation method, it is a general practice to employ a method in which acetone is added to total lipids to precipitate complex lipid (phospholipid) (insoluble portion), after which the insoluble portion is washed with ether to remove glycerophospholipid, and the residue is taken as a crude sphingolipid fraction. This fraction contains not only sphingomyelin, but also glycerosphingolipids such as cerebroside.
On the other hand, it is known that the phospholipid of chicken skin contains a great deal of human-form sphingomyelin and plasmalogen-form glycerophospholipid.